Source term for performance of assessment of spent fuel as a waste form

Detalles del proyecto

Coste total:

Aportación de la UE:

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Régimen de financiación:

CSC - Cost-sharing contracts

Objetivo

The direct disposal of spent nuclear fuel as a waste form is currently under consideration in many EU countries. A requirement in evaluating disposal safety is the characterization of the source term for longlived radionuclides upon contact with repository relevant aqueous media; and materials. Research programs worldwide have provided significant insight into the spent fuel dissolution process. Due to uncertainties in interpretation and quantification of release controlling processes and due to the long time spans to be considered, existing experimental data cannot be extrapolated reliably to relevant disposal times and disposal configurations. The proposed work aims at reducing these; uncertainties in source term quantification by combining experimental and modeling approaches. Emphasis is on real spent U02 and MOX fuel. Suggested experiments are directed simultaneously towards (I) basic understanding of the mechanism and quantification of rates of fuel matrix dissolution in three relevant geological environments (salt, clay, granite) under a variety of environmental conditions (redox conditions, S/V, temperature, pH, pC02) (II) understanding and quantification of solubility, sorption and coprecipitation equilibria for individual radionuclides, (III) understanding of the effect of near field materials (iron, iron corrosion products, bentonite, host rock) on reaction rates and radionuclide release. Five types of experiments are suggested: dissolution tests, codissolution tests with near field materials, precipitation tests and co-precipitation tests and electrochemical tests with U02 or spent fuel electrodes. Dissolution and co-dissolution tests will provide rate data and may allow the system to approach quasi-equilibrium conditions (saturation, steady state...) from initially undersaturated conditions while precipitation and co-precipitation tests should facilitate an approach towards quasi-equilibrium from oversaturation. Electrochemical tests will allow to distinguish between electrochemical and chemical dissolution, and between oxidative and non-oxidative dissolution. Comparison of the high radiating with low-radiating materials (SIMFUEL, U02) shall help to identify radiolysis effects. Model development has two major goals: (a) it shall provide a basis to describe the multitude of new experimental results together with literature data in a consistent and unambiguous manner and (b) for the various geological formations, reliable source terms shall be formulated, useful for performance assessment of spent fuel as a waste form. Model development comprises modeling of the geochemical reaction path and of actinide chemistry, of dissolution kinetics and coprecipitation phenomena and radiolysis effects. The model will be scaled-up to repository configurations and disposal times to derive a source term for long-term radionuclide release from spent fuel.